Abstract:

In this Master’s Thesis the anaerobic treatment of Stora Enso Heinola Fluting mill condensates was examined and its benefits for the mill assessed. The anaerobic treatment of the condensate could reduce the load towards waste water treatment plant and produce biogas. The secondary condensate from semi-chemical pulp and board mill carries high load of fast degrading organic matter especially acetic acid, thus it could be suitable for biogas production.

In the 10-day laboratory scale batch tests the biogas production rate of only 0.19 m3/kg COD, instead of expected rate of 0.38 m3/kg COD, was measured with condensate. The methane-carbon dioxide ratio at the time was 1.7. It was observed that the pH remained optimal without any neutralizing, as the organic acids were decomposed during the process. In the following phase the condensate, and the mixture of the bio sludge, condensate and the bio sludge filtrate was treated anaerobically for one and a half month in a completely mixed, semi-continuous batch reactor. In the arrangement feeding of the waste water was set continuous with a timer while the removal was performed batch wise by first separating the waste water from the biomass with a centrifuge. All samples presented a methane-carbon dioxide ratio greater than three while the proportion of methane from the composed biogas was ca. 75%. Despite of the acidity of the feed the pH remained between 6.8 and 7.5 without neutralizing. With the condensate, the COD reduction measured 75 % and somewhat lower with other fractions, except for the sample containing pre-treated bio sludge for which the reduction was ca. 85 %. According to the literature study, for the detailed dimensioning and cost assessment, IC, EGSB and AnMBR technique were selected as anaerobic process solutions. The selection between five different anaerobic condensate treatment processes was made by comparing different variables describing efficiency and cost. With an IC or EGSB reactor the production of biogas was estimated to be between 5160 and 6240 m3/d. Information about anaerobic treatment of condensate with different process solutions were received from various equipment suppliers and these proposals were used in order to assess possible laboratory measurements as well as when evaluating the benefits for the mill. Impact assessment took into account the anaerobic treatment’s energy and chemical consumption impacts to existing waste water treatment plant at Heinola Fluting Mill. All this was done parallel taking into account the potential biogas applications either for energy production at the mill site, as a vehicle fuel or selling the gas as unpurified.

According to the results, the condensate is feasible for anaerobic treatment bringing savings for the mill as the organic load at the waste water treatment plant is reduced by 40%. According to the Thesis, either IC or EGSB technique would be feasible for Heinola Fluting Mill as an anaerobic process. The best option at the moment for the use of biogas is the energy production for internal consumption which would reduce the use of peat.Tässä diplomityössä tarkasteltiin Stora Enson Heinolan Flutingtehtaan lauhteiden anaerobista käsittelyä ja sen tuomia hyötyjä tehtaalle. Lauhteen anaerobinen käsittely pienentäisi jätevesilaitoksen kuormitusta ja tuottaisi biokaasua. Puolikemiallisella sellu- ja kartonkitehtaalla muodostuvassa sekundäärilauhteessa on paljon nopeasti hajoavaa orgaanista ainetta, erityisesti etikkahappoa, joten lauhde voi soveltua hyvin biokaasun tuotantoon.